The Nellore district in the Southeastern part of Andhra Pradesh is one of the nine coastal districts of the state. The microtremor activity in the district started in October 2015 and continued up to July 2016. During this period, a few hundred tremors were recorded by the temporary seismic network installed locally by the CSIR-NGRI. Apart from the data recorded by this network, those from the semipermanent seismic stations at Racherla, Addanki, Cuddapah, Srikalahasti and Srisailam were used in the study. The Nellore Green Schist Belt (NSB) in the southern peninsular shield consists of several geological and geochemically distinct tracts of deformed volcano sedimentary successions:
Well-located microtremors define a narrow epicentral zone aligned along Nellore–Badvel Fault oriented in the NW–SE direction and TBSZ, which is marked by several major lineaments parallel to CEMT12. These are responsible for the recent ongoing swarm activity in Chakalakonda–Kothapeta areas situated in the NSB. The seismogenic depth is found to be shallow (<9 km). Focal mechanism studies reveal strike–slip faulting. Interestingly, correlation of rainfall data with the timing of seismicity, causally supports the hypothesis of hydroseismicity. Pore pressure diffusion guided by a permeable fracture zone in this region, is a plausible mechanism for this swarm type hydroseismicity. A comprehensive analysis leads to the inference that earthquake swarm nucleation is a result of time-dependent stress changes induced by water infiltration due to excess rainfall in areas straddling the seismic zone, like Varikuntapadu Mandal in Nellore, in addition to the constant tectonic stress prevailing in the region.
Plot of epicentres on image interpretation of landsat TM11. Some of the major lineaments (shown with numbers) are: (1) Cuddapah Boundary Thrust, (2) Nallamalai/ Rudravaram Line, (3) Gani-Kalva Fault, (4) Nellore-Badvel Fault, (5) Karakambadi Fault, (6) Terrane Boundary Shear Zone (TBSZ) and (7) Gundlakamma Fault.